Acid-in-oil emulsions are typically used to stimulate or enhance hydrocarbon production in existing carbonate reservoir rock formations, such as limestone, dolomite or calcareous-magnesium. Typically, the emulsified acid enters the formation and where employed successfully creates a barrier causing the acid to release slowly at a distance from the well-bore. The reaction of the released acid with the formation rock takes place simultaneously at different places inside the formation, resulting in channels that are joined together to form continuous wormholes. When pumping the acid-in-oil emulsions through steel tubing and piping, a corrosion inhibitor is usually added to reduce the corrosive effects of the acid. In operation, the corrosion inhibitor coats the steel surfaces as the emulsion is pumped into the well-bore and the surrounding rock.
In current practice, the oil emulsion consists of two phases. They are the internal phase formed of acid with corrosion inhibitor added to it, and the external phase formed of oil with an emulsifier. The currently employed ratio used in the field is 30% oil and 70% acid. Acid-in-oil emulsions are prepared by mixing the oil with an emulsifier and then gently adding the acid which has been mixed with the corrosion inhibitor to the emulsified oil phase. The oil is known as the disperse phase and the mixture of acid and corrosion inhibitor as the inner phase.
Thus, the corrosion inhibitor which is dissolved in the acid phase is encapsulated by the oil which contains the emulsifier. The corrosion inhibitor, accordingly, is disposed internally with the emulsion droplet which limits its ability to readily disperse on the metal surfaces to create a protective film.
Groote, U.S. Pat. No. 1,922,154 was the first to disclose the use of emulsified acid in the oil industry. Groote discloses removal of formation damage from carbonate rocks with an aqueous acid solution emulsified in a suitable medium that effectively protects the metallic parts of the well from damage by the acid in the solution, while the solution is being pumped into the well. Groote employed hydrochloric acid, nitric acid and a mixture of the two acids to prepare his emulsion. Crude oil and coal tar distillates, such as naphtha and carbon tetrachloride were used as dispersing fluids. Sulfonic acid was used as the emulsifying agent. The feature described in the '154 patent put the pairing the emulsion as similar to current oil field practices. Approximately 2% to 5% of the emulsifying agent is added to crude oil which forms continuous phase and the acid containing the corrosion inhibitor is added to the mixture at a ratio of 33.3% acid to 66.7 parts of crude oil by volume ratio.
Bland, U.S. Pat. No. 7,354,886 discloses the transfer of a corrosion agent downhole by encapsulating it in three phases to release it at the target zone.
U.S. Pat. No. 6,464,009 discloses a method of delivering and releasing a corrosion inhibitor downhole by a pumpable composition consisting of multiple phases.
Martin et al. U.S. Pat. No. 5,753,596 discloses a method of mixing corrosion inhibitor consisting of incorporating thiophosphates consisting of both oxygen and sulfur, pyrophosphates, containing both oxygen and sulfur or a mixture into an oil phase of an emulsion.
French, et al. (U.S. Pat. No. 5,027,901) discloses a method of mixing a corrosion inhibitor, adding it to a discontinuous phase (kerosene) and pumping this discontinuous phase to the well annuals before mixing with a continuous phase (produced water) inside the well.